The Influence of Nitrate on Selenium in Irrigated Agricultural Groundwater Systems

Selenium (Se) contamination of groundwater is an environmental concern especially in areas where aquifer systems are underlain by Se-bearing geologic formations such as marine shale. This study examined the influence of nitrate (NO3) on Se species in irrigated soil and groundwater systems and presents results from field and laboratory studies that further clarify this influence. Inhibition of selenate (SeO4) reduction in the presence of NO3 and the oxidation of reduced Se from shale by autotrophic denitrification were investigated. Groundwater sampling from piezometers near an alluvium-shale interface suggests that SeO4 present in the groundwater was due in part to autotrophic denitrification. Laboratory shale oxidation batch studies indicate that autotrophic denitrification is a major driver in the release of SeO4 and sulfate. Similar findings occurred for a shale oxidation flow-through column study, with 70 and 31% more reduced Se and S mass, respectively, removed from the shale material in the presence of NO3 than in its absence. A final laboratory flow-through column test was performed with shallow soil samples to assess the inhibition of SeO4 reduction in the presence of NO3, with results suggesting that a concentration of NO3 of approximately 5 mg L-1 or greater will diminish the reduction of SeO4. The inclusion of the fate and transport of NO3 and dissolved oxygen is imperative when studying or simulating the fate and transport of Se species in soil and groundwater systems.